Question

I make a benchmark like this :

for (int i = 0; i < 1000 * 1000; ++i) {
    long var = System.nanoTime();
}

it takes 41 ms in my computer with jdk6.0

the follow code only costs 1 ms!!!

for (int i = 0; i < 1000 * 1000; ++i) {
    System.nanoTime();
}

I think may be that time is costed by long var , so I make a test like this :

 for (int i = 0; i < 1000 * 1000; ++i) {
    long var = i;
}

it only costs 1 ms!!!

So,Why the first code block is so slow?
I’m a Chinese. Sorry for my poor English!

Answer 1

It really depends on how you run your benchmark. You most likely get <1ms runs because the JVM is not really running your code: it has determined that the code is not used and skips it:

for (int i = 0; i < 1000 * 1000; ++i) {
    long var = i;
}

is equivalent to

//for (int i = 0; i < 1000 * 1000; ++i) {
//    long var = i;
//}

and the JVM is probably running the second version.

You should read how to write a micro benchmark in Java or use a benchmarking library such as caliper.